A diagnostic ultrasonic imaging system for medical use may be utilized to form images of tissues of a human body by electrically exciting a piezoelectric transducer element or an array of elements, which then generates short acoustic pulses, conventionally within the ultrasonic frequency region, that are caused to travel into the body. Echoes from tissues are received by the ultrasonic transducer element or elements and are converted into electrical signals. Electrical signals are amplified and used to form a cross sectional image of the tissues. Echographic examination is also employed outside of the medical field.
An ultrasonic transducer probe is used to generate a beam of broadband acoustic waves that is acoustically coupled from a front portion of the probe, through an acoustic lens, and into a medium of interest, such as a human body. The acoustic lens is used to focus the beam of acoustic waves. Within the medium of interest, various structures will reflect a degree of the beam of acoustic energy. Weak reflections are received at the front portion of the ultrasonic probe. By analyzing the relative delay and intensity of weakly received acoustic energy, an imaging system may be used to extrapolate an image of the structures within the body.
Since the acoustic wave energy that enters the medium of interest is only weakly reflected back to the ultrasonic probe by the structures within the medium, it is important to reduce any acoustic wave energy reflected by a rear portion of the probe. If a portion of the acoustic waves that are generated by the probe is directed rearwardly and is reflected by the rear portion of the probe, a first unwanted acoustic signal will be transmitted into the medium of interest. Similarly, if a portion of the weakly reflected wave energy that is received by the probe is transmitted through the probe and then reflected by the rear portion, a second unwanted acoustic signal is produced. As a result, distortions will occur within the extrapolated image.
An approach to reducing reflections from the rear portion of the probe is to attach an acoustically damping support body to the rear face of the probe. The damping support body is typically referred to as the backing layer. In order to further reduce reflections, a layer of matching material may be coupled between the piezoelectric layer and the damping support body. For example, an ultrasonic transducer may include a piezoelectric layer of lead zirconate titanate (PZT) having an acoustic impedance of 33.times.10.sup.6 kg/m.sup.2 s, a dissimilar acoustic layer of silicon having an acoustic impedance of 19.5.times.10.sup.6 kg/m.sup.2 s, and a damping support body of epoxy resin having an acoustic impedance of 3.times.10.sup.6 kg/m.sup.2 s. The silicon layer is used to provide an improved acoustic impedance match between the relatively high acoustic impedance of the PZT layer and the relatively low acoustic impedance of the epoxy resin support body. Typically, the impedance matching layer has a thickness that is one-quarter wavelength of the resonant frequency of the transducer.
While the use of a back matching layer and an acoustically damping support body reduces the reflections at the rear of the device, reflections nevertheless occur, since a degree of impedance mismatch still exists. Furthermore, a thin layer of adhesive is applied to bond each of the layers, thereby creating undesirable adhesive bond lines. The thickness of the bond line may vary within a range of 2 microns to 25 microns, thus becoming an additional source of acoustic wave energy reflection. Another concern is that the bonding process steps sometimes create manufacturing difficulties. For example, during manufacturing it is difficult to ensure that no voids are introduced into the adhesive. Such voids impair operation of the probe. Furthermore, reliability of ultrasonic transducers is adversely affected by differing thermal expansion coefficients of the layers. Over time, some of the adhesive bonds may lose integrity, resulting in transducer elements that no longer provide efficient acoustic coupling. Yet other concerns are that the bond lines may impose limits on operational performance of the probe at high acoustic signal frequencies, such as frequencies above 20 MHz, and that the availability of suitable materials having the desired acoustic properties for the required impedance matching layer might be limited.
Acoustic impedance matching is also important at the front portion of the ultrasonic device. Efficient acoustic coupling to the medium of interest reduces the reflection at the interface of the acoustic device and the medium. Minimizing reflection is important both in transmitting wave energy from the device to the medium and in receiving the energy from the medium for imaging tissue structures and the like. As previously noted, a PZT transducer layer has an acoustic impedance of approximately 33.times.10.sup.6 kg/m.sup.2 s. The acoustic impedance of PZT is poorly matched with the acoustic impedance of human tissue, which has a value of approximately 1.5.times.10.sup.6 kg/m.sup.2 s.
One technique for reducing energy reflection at the front portion of the device is to utilize a front impedance matching layer having a thickness of one-quarter of the wavelength of the operating frequency of the transducer and having an acoustic impedance equal to the square root of the product of the acoustic impedances of the device and the medium. The front matching layer is bonded to the piezoelectric material in the same manner as the back matching layer. Thus, the same concerns exist at the front surface, e.g. the selected bonding material may create a layer that tends to interfere with the acoustic wave transmission, especially at relatively high ultrasonic frequencies, and there are reliability issues, such as adhesive debonding.
Another approach to improve acoustic coupling is described in "New Opportunities in Ultrasonic Transducers Emerging from Innovations in Piezoelectric Materials," W. A. Smith, SPIE (Society of Photo-Optical Instrumentation Engineers), Volume 1733 (1992), pages 3-26 and in "Modeling 1-3 Composite Piezoelectrics: Thickness-Mode Oscillations," W. A. Smith, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Volume 38, No. 1, (January 1991). Smith describes forming a piezocomposite wave-generating layer that is a combination of piezoelectric ceramic and a passive polymer. In what is described as the most widely used method to make the 1-3 piezocomposites, a block of piezoceramic material is subjected to a dice-and-fill technique in which two sets of cuts are formed at right angles to each other. A polymer is then cast into the cuts. After polishing the resulting structure to the desired thickness, electrodes are applied to the opposed surfaces and the ceramic is poled to align ferromagnetic domains. The resulting structure is one in which the wave-generating layer has an array of full-thickness piezoceramic pillars that are spaced apart by the polymer. Consequently, the structure has an acoustic impedance that is lower than the acoustic impedance of bulk piezoceramic, reducing the acoustic impedance mismatch with the medium of interest.
While composite materials provide some improved acoustic coupling to various media, there are difficulties in electrically sensing reflected acoustic waves received by such composites. The dielectric constant is relatively small. For example, for a composite that is 50% polymer and 50% piezoelectric ceramic, the dielectric constant measurable between electrodes of the high polymer is approximately half of that which is inherent to the piezoelectric ceramic. A much higher dielectric constant is desirable, so that a higher capacitive charging is sensed by the electrodes in response to received acoustic waves. A higher dielectric constant also provides an improved electrical impedance match between the ultrasonic device and components of the imaging system that are electrically coupled to the device.
What is needed is a method of fabricating a transducer device that provides enhanced operational performance.